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We prepared isolated subsurface Co atoms below the (100) surface of Copper by co-deposition of host metal and impurity atoms from two e-beam evaporators on a carefully cleaned single crystal. The influence of the buried magnetic impurities on the surface local density of states (LDOS) was investigated using Scanning Tunnelling Spectroscopy at 6K. Co atoms lying up to 20 monolayers below the surface were identified from their highly anisotropic bulk state LDOS oscillations. Due to the nonspherical Fermi surface of Cu the electrons propagate in preferential directions perpendicular to flat areas of the host metal Fermi surface ("electron focusing"). This causes the electron density to be influenced only in narrowly confined directions from the impurity. The constant angle of electron propagation results in an expanding diameter of the observed pattern with increasing impurity depth. The area within this cone is much less influenced. By obtaining the single electron propagator G0 from the host metal band structure, we calculate the surface LDOS and obtain a good agreement with the observed STM patterns. A comparison between calculation and experiment allows us to determine the depth and the t-matrix of the impurity atoms. In multi-bias topographies and dI/dU- spectroscopies the LDOS patterns show a distinct energy dependant phase shift around zero bias and Fano-lineshapes with different asymmetries. This is a direct consequence of resonant impurity scattering due to the Kondo effect. The data allow to discriminate between changes in the surface LDOS that are caused by dispersion and those due to a change in the scattering behaviour (reflectivity, phase shift) of the impurity. Since the latter is directly connected to the impurities spectral function, the Kondo-resonance can be clearly identified even for impurities lying as deep at 10ML below the surface. As the distance from the impurity approaches the Kondo coherence length ξK=vF/(kBTK), which describes the extension of the Kondo screening cloud, the influence of the Kondo-resonance and phase-shift on the LDOS gets more and more masked by the impact of the dispersion. |